Where can I find an AC generator for a hydroelectric power.?

AC generators are difficult to synchronize, as you have to match both voltage and frequency, so you are better off with only one. You may be better off with one or several alternators as the DC output is easy to regulate and you don't have to worry about phase and frequency. You can then use an inverter to convert that 12 (or 24 or 48) volts DC to the AC voltage you need. Alternators are also easy to find, as you can get a large truck or bus alternators that will supply many thousands of watts at 24 volts DC. It's very difficult to find generators online, as the search returns mostly motor-generator sets. You don't mention voltage or number of phases you want for output?? .

One idea is to use an induction motor as the generator, as these are readily available. The motor is about the same power as the generator, so 50KW. It has to be connected to a grid for excitation, or to itself using large excitation capacitors. These are not easy to obtain that I know of. A 4 pole induction motor generates 50Hz @ about 1500RPM. See the first and last links for a starter, then look up more yourself. I have to add that the generator needs to be matched to the RPM and torque of the turbine in some way. I would think the turbine is actually more difficult to build yourself. The link below shows a source of turbines and generators and other components. This system also includes the necessary control for governing (regulating) the speed of the turbine. The dump load mentioned is a minimum load, which is for better control of the turbine at very light loads. It is important to verify that you will have the required power (water head and flow) before you spend money. See common questions in the third link. There are usually permits, environmental impact studies etc required too.

One idea is to use an induction motor as the generator, as these are readily available. The motor is about the same power as the generator, so 50KW. It has to be connected to a grid for excitation, or to itself using large excitation capacitors. These are not easy to obtain that I know of. A 4 pole induction motor generates 50Hz @ about 1500RPM. See the first and last links for a starter, then look up more yourself. I have to add that the generator needs to be matched to the RPM and torque of the turbine in some way. I would think the turbine is actually more difficult to build yourself. The link below shows a source of turbines and generators and other components. This system also includes the necessary control for governing (regulating) the speed of the turbine. The dump load mentioned is a minimum load, which is for better control of the turbine at very light loads. It is important to verify that you will have the required power (water head and flow) before you spend money. See common questions in the third link. There are usually permits, environmental impact studies etc required too.